(1) Field of the Invention
The present invention relates to the fabrication of LCD integrated circuit devices, and more particularly, to a method of bonding pad definition which allows for the formation of a highly reflective pixel plate in the fabrication of LCD integrated circuit devices.
(2) Description of the Prior Art
Liquid crystal displays (LCD) have been used in the art for such applications as computer and television screens. Typically, the topmost level fabricated on a bottom substrate is a series of metal pixels, 19.times.19 microns in area. There are typically 1024.times.768 pixels having a spacing of 0.7 microns between each pixel. The liquid crystal display layer is built on this array of metal pixels and enclosed by a top substrate comprising a transparent material.
FIG. 1 illustrates a LCD integrated circuit device of the prior art. Bottom semiconductor substrate 10 is illustrated. Gate electrodes and source and drain regions having connections to a first metal layer would be formed in and on the semiconductor substrate. Only the second metal layer 22 and above are shown in this illustration. Second metal lines 22 are formed on the surface of the substrate. An insulating layer 26 is deposited over the metal lines. Tungsten plugs 28 are formed within openings through the insulating layer to the metal lines 22. A third level of metal lines 32, usually an aluminum alloy, are formed overlying a barrier layer 30, typically titanium nitride. The barrier layer typically has a thickness of about 1000 Angstroms and the metal lines 32 have a thickness of about 5000 Angstroms. The aluminum alloy is separated and protected by a passivation layer, typically 1000 Angstroms of undoped silicate glass (USG) 34 followed by 2500 Angstroms of silicon nitride 36. The liquid crystal material 52 lies between the passivation layer and the top substrate 56. Sometimes the pixel formation is in a fourth metal layer rather than the third metal layer.
Since the reflective metal lines 32 forming the pixels are at the same level as the bonding pad, not shown, the thickness of the metal layer is typically the thickness required for good bondability; i.e. about 5000 Angstroms. However, a very thin aluminum layer is preferred for maximum reflectivity of the pixels, about 1000 to 2000 Angstroms. Another issue in forming the pixels and bonding pad simultaneously is that the bonding pad via opening is typically much larger than the via openings for the pixels. The tungsten plug process requires via openings to be close to equal in size. Otherwise, tungsten residue in the large opening will result in poor metal adhesion.
The deep trenches 60 between the pixels caused by the thick aluminum layer 32 cause the LCD material to become trapped between the pixels. Due to the sequencing effect required for image formation, the pixels will have differential voltages. If two adjacent pixels have slightly different voltages, the LCD material trapped between them may light up undesirably. This is termed "cross-talk."
There are a number of patents in the field of LCD's. For example, U.S. Pat. Nos. 5,537,234 to Williams et al, 5,705,424 to Zavracky et al, and 5,056,895 to Kahn teach various LCD fabrication techniques. U.S. Pat. No. 5,591,480 to Weisman et al teaches a method of forming a metal pattern having multiple metal thicknesses and composition. However, none of these patents address the issue of bonding pad thickness versus optimal pixel thickness.